Heat radiating coatings



June 5, 1961 T. A. STERNBERG 2,987,423

HEAT RADIATING COATINGS Filed Sept. 26, 1958 2 Sheets-SheetI 2 lpg?. 2.

INVENTOR. THE m @RE JSTEENBFRB United States Patent() 2,931,423. HEAT RADIAUNG coArrNGs Theodore A. Sternberg, Basking Ridge, NJ., assignor-to-v Radio Corporation of America, a corporation o f-Delaware.

Filed Sept. 26, 1958, Ser. No. 763,704 14. Claims. (Cl. 117-217) This invention relates to. a methodgof coating; metal articles to improve their ability to radiateV heat.

in vacuum tubesV audY thelike.

Electron tubes: and the. like areusually provided with, a cathode having a coated surface whiclrV readily emits,

electrons, and an anode which collectsthe emittedv electrons.. The anode Iis bombardedby electronsduring the operationY of the tube and therefore-heats up rapidly.

This heat must bedissipated before the temperature of ,the-k tube.becomes too highV for eicient operation, due,A to gas,-

anodes are therefore treated to provide rough dark sur-` faces, which are known to be more etlicient heatradiators than smooth bright surfaces.

One method of obtaining an eicient heat-radiatingv surface is to coat the metallic varnzsde with aluminum, thus obtaining a smooth bright surface, andsubsequently heat treating the anode to convertthe bright surface, which is, a poor heat radiator into a rough dark surface Whichis The converting of. a smooth.-

an efiicient heat radiator. bright surface into a roughdarksurface iskuown` in the. vacuum ltube art as alitizing.

Vacuumtube parts such as anodes arecommonly made ofl ferrous or nickel alloys. Nickel-coated alloys suchas cladsteels and nickel-iron,alloysarealso utilized for this purpose. These. base materials are usually` supplied as rolls of flexible strip, and have previously been coated withaluminum either by. dipping in, molten` aluminum, or1

by cladding, for example by cold rollingV steelstrip between two strips of aluminum. The aluminum coating thus formedA is a poor heat radiator, being smooth and bright. The strip is then formed into anodes and various other electron tube parts, such as plates, beam platesand.

grid collars, and thereafter heat treated' to convert the bright aluminum coatinginto arough dark surface which is an excellent heat radiator. Howevensati'sfactory aluminum-coated strip is relatively expensive, since the dip process, which is similar to that described in U.S. Patent 2,110,893, is subject -to oxidation andrequires special ambients. The clad materials requireI heavy presses and high pressures, and even then the aluminum doesv not; adhere to the strip unless both have been carefully cleaned, pickled, brushedand sand blasted. Moreover, clad material is not readily available from domestic sources, so that imported aluminum-coated material must be relied upon.

An object ofV this invention isvthe provision of an improved method of forming an improved heat-radiating surface on a metallic base.

Another object is. the provision of; an inexpensive process for forming an eicient heat-reducing surface on a metallic base.

Still another object is the.- provision of an improved method formaking aluminum-coated strip.

Yet another Objectis the provision of a. method of making alitized metal parts from domestic materials.

These and other objects are provided by the. instant invention, in which a base materialr such as metallic strip selected from the group consisting of nickel, iron, ferrous alloys, nickel alloys, and' nickelcoated alloys, is coated The processzof theinstant inventionisparticularly applicable. tov the .manufacture of coated; iron. 0,1'. nickel parts for usev with a iirst slurry comprising a material selected from ,.th

group consisting of iron oxide, nickel oxide, and mixtures of iron and nickel oxides. The iirst slurry is. dried, and, the oxide is then reduced to the metal. The spongyre.- ducedmetal is thereafter sintered, and a second slurry comprising comminuted aluminum suchasV nely divided,v powder-.or akes is applied to thesintered reduced metal. Next the. second slurry is dried, leaving an aluminumcoated surface over the. base. Last, the base material is alitized by heat treatment in vacuum to producea rough dark surface whichis .an etlcient, heat radiator. The last. step, is preferably performed, after thebasematerial has., been formed into an electron tube part and has been inf. serted in the tube.v

The invention. and. its. advantages.. will beI describediu. greater. detail with reference to. the accompanyinadrawing, .iu which:

FIGURE 1 is` allow Sheet showing the. various. steps. ofthe alitizing process;

FIGURE 2 is a schematic diagram of one type off apparatus` useful in performing the processV of thisr inve'ntion.

As shownrin FIGURE l, the rst stepin the process. is the preparation of the first slurry, which comprises ariI oxide of `iron or nickel, or a mixture of these oxides. A,

mixture containing about 1-20% by weight nickel oxide, and the balance iron oxide, has been. found particularly s uitable. Any of the three oxides of iron (FeOgFezOh FeaOg) and the two oxides of nickelI (NiO, Ni203)` may be utilized. The powdered oxide is mixed with an or. ganic binder, such as nitrocellulose or carboxymethyl-- cellulose or the likeand an organic solvent whichA is' capable of dissolving thebinder. The proportions of, theseV ingredients are adjusted sothat the slurry contains from itl-% by weight ofthe oxides. AtY higher concentrations, the slurry becomesan extremely thick paste. A concentration ofabout 20% by weight ofthe oxide in the slurry has-beenfound suitable for many applications.

The method may be best understood with Areference to aparticular example. In this example, a mixture ofv 99% ferrie oxide Fe203 and 1% nickel oxide Ni() (by weight)-is used to make the rst slurry. Ten pounds of'I the mixed oxides which have been powdered to pass-1507 37 S'mesh are ball milled for about 3fhours withl22()A cc.

of: anv organic solvent suchY as an ester ora ketone. In' this example, the solvent is butyl acetate. A` 4 gallon mill containing 28` pounds of three-quarter inch 'steel balls is utilized. The powder has a consistency (North Standard) of"4'.`0`before the mixing, and 6.0' (semi-paste) after mixing. Another 5 50 cc. of'butylV acetate-is added, and milling is continuedv for about 2 hours until the mix#Y ture reachesapasty consistency (North Standard 6.75). Now 190 cc. of butyl acetate and 1610 cc. of nitrocellulose (40 sec cotton) is added, andthe mixture is balk milled to a paint-like consistency.

The firstvslurry isthen applied rto the base, which may be nickel, iron, nickel alloys, ferrous alloys such as steel, or nickel: cladsteels. In this example, the base consists of steelstrip about 7 inches wide and about 5 mils thick. These dimensions are not critical, and 'the strip may for example be 4 to 10 mils thick. The apparatus shownin FIGURE 2 may be utilized to perform the 'process in a. continuous manner, the strip being spooledthrough the apparatus at aboutZ to 7 feet per-minute. The speed. is,i adjusted in accordance with,I the apparatus dimensions. and the temperatures of the driers and ovens. Irlnthis,l example, the strip 10 travels at. the. rate of 5 feet per minute from` roll 11 into applicator 12, where it receives a coating of the first slurry 143x. The coating is about the 200 C. In this example, the temperature of the drierv is kept at about 160 C. A dried layer of the mixed iron oxide and nickel oxide is left on the surface of the strip.

The next step in the process is the reduction of the Oxide coating to the metal by heating the strip in a reducing atmosphere at a temperature of about 800 to 1000 C. The residue of the organic binder is reduced and volatilized during this step. =In this example, the strip 10 is passed through a hydrogen furnace 15 which contains two sets of heaters 16 and 16'. A thermocouple 17 between the two sets of heaters and about 2 inches away from the strip is used to monitor the furnace temperature. The power supply is regulated so that a temperature of about 975 C. is maintained in the center Vof the furnace 15. A stream of hydrogen is passed into the furnace at inlet 18 and leaves at outlet 19. The pressure of hydrogen is maintained slightly above atmospheric, in order to exclude air from the furnace. Other reducing ambients such as forming gas may be utilized. As the strip 10 passes through the lower or reduction zone of the furnace, the oxide Ycoating is reduced to the metal.

The reduced metal, whether iron, nickel, or a mixture of these, is rough and spongy, but does not adhere well to the strip. For this reason the spongy metal is sintered by heating the strip at a temperature below the melting point of the strip and the spongy metal layer. A second furnace is not required, since this step may be performed under the same conditions as the previous reduction step. The strip 10 passes between the second set of heaters 16' in the furnace 15, and is thereby heated sufiiciently to sinter the spongy metal and improve its adherence to the strip.

. The strip 10 is next rolled through applicator 20, where it is coated with the second slurry 21, which comprises comminuted or nely divided aluminum together with an organic binder and a solvent for the binder. The aluminum may be in the form of a powder or akes, and the concentration of the aluminum in this slurry may vary from about to 20% by weight. In this example, the second slurry 21 is prepared by mechanically mixing or ball milling 5 pounds of ake aluminum with 4530 cc. of nitrocellulose (40 sec cotton) as binder and 13,590 cc. of butyl acetate as solvent. This mixture contains about by weight aluminum, although slurries with about 5% to 20% by weight aluminum may also be utilized. The second slurry 21 adheres to the sintered metal surface on the strip 10, and forms a coating about .1 lto .5 mil thick.

The strip is again dried in air. Since drying at room temperature is too slow, the strip 10 is passed through Va second drier 22, which is maintained at 10U-200 C. In this example, the drier 22 is kept at about 160 C.

The strip may then be rolled and stored, or sent directly to a slitting machine where it is cut into portions of appropriate size. The portions are then formed into the desired parts, for example electron tube components such as anodes, beam plates, and grid collars.V

For vacuum tube applications, it is desirable to reduce the residue of the organic binder on the surface of the parts so as to prevent the subsequent evolution of gases when the parts are used as vacuum tube components. Reduction may be accomplished by heating the parts in a reducing ambient for a few minutes at a temperature high enough to insure rapid reduction of the binder. In this example, the parts are heatedin a'hydrogen furnace for about 2-3 minutes at about 600 C. This heating cycle is not sufiicient to alitize the aluminum-coated surfaces of the parts, which remain smooth and bright.

. theparts are anodes, which areVinsertegl-into glass tubes.

While the tubes are being evacuated, the tube components are heated to about 725-1000 C. In this example, the anodes are heated to about 850 C. during the alitizing step. The surface of the anode changes color during this step from smooth shiny bright, which is a poor heat radiator, to a rough dark slate gray, which is an ecient heat radiator.

There has'thus been described -an improved method of providing an eicient heat-radiating surface on metal parts such as vacuum tube anodes. The method is not limited to electron tube components, and may be applied Whereever a good heat-radiating surface is required. `It will be understood that various modilications may be made in the process without departing from the spirit and scope of the instant invention. For example, the slurries may be made with other binders such as potato starch and corn starch, and with other solvents such as ethyl acetate and propyl acetate.

What is claimed is:

1. A method of providing a heat-radiating surface on a base selected from the group consisting of nickel, iron, ferrous alloys, nickel alloys, and nickel-coated alloys, which comprises coating said base with a rst slurry comprising a material selected from the group consisting of iron oxide, nickel oxide, and mixtures of iron and nickel oxides, drying said iirst slurry, reducing said oxide to the metal, sintering said metal to said base, applying to said sintered metal a second slurry comprising comminuted aluminum, drying said second slurry, and subsequently heating in vacuum to produce a heat-radiating surface.

2. A method of providing a heat-radiating surface on steel strip, which comprises coating said strip with a first slurry comprising a material selected from the group consisting of iron oxide, nickel oxide, and mixtures of iron and nickel oxides, drying said first slurry, reducingv said oxide to the metal, sintering said metal to said strip, applying to said sintered metal a second slurry comprising comminuted aluminum, drying said second slurry, and subsequently heating said strip in vacuum to produce a heat-radiating surface on said strip.

3. A Vmethod of providing a heat-radiating surface on steel strip, which comprises coating said strip with a first slurry comprising ircn oxide, heating said strip to dry said first slurry, reducing said oxide to iron, sintering said iron to said strip, applying to said sintered iron a second slurry comprising comminuted aluminum, heatingvsaid strip to dry said second slurry, and subsequently heating said strip in vacuum to produce a heat-radiating surface on said strip.

4. A method of providing a heat-radiating surface on a base selected from the group consisting of nickel, iron, ferrous alloys, nickel alloys, and nickel-coated alloys, which comprises coating said base with a first slurry comprising iron oxide and nickel oxide, the weight of the nickel oxide being l-20% of the Weight of the iron oxide, heating said base to dry said rst slurry, reducing said oxides to the metal, sintering said metal to said base, applying to said sintered metal a second slurry comprising comminuted aluminum, heating said base to dry said second slurry, and subsequently heating said base in vacuum to produce a heat-radiating surface on said base.

5. A method of providing a heat-radiating surface on a base selected from the group consisting of nickel, iron, `ferrous alloys, nickel alloys, and nickel-coated alloys,

which comprises coating said base with a first slurry com# prising an organic binder, a solvent for said binder, and a material selected from the group consisting of iron oxide, nickel oxide, and mixtures of iron and nickel oxides, heating said base to dry said first slurry, reducing said oxide to the metal, sintering said metal to said base, applying to said sintered metal a second slurry compris- -ing an organic binder, asolvcnt for( said binder, and

anexa-2e Vcomminuted aluminum, heating said base to dry said sec- 'ond slurry, and subsequently heating said base in vacnum to produce a heat-radiating surface on said base.

6. A method of providing a heat-radiating surface on a base selected from the Agroup consisting of nickel, iron, ferrous alloys, nickel alloys, and nickel-coated alloys, 'which comprises coating said base with a first slurry comprising an organic binder, a solvent for said binder, and a material selected from the group consisting of iron oxide, nickel oxide, and mixtures of iron and nickel oxides, said slurry containing to 80% by weight of said oxide, heating said base to dry said rst slurry, reducing said oxide to the metal, sintering said metal to said base, applying to said sintered metal a second slurry comprising an organic binder, a solvent for said binder, and comminuted aluminum, said aluminum powder comprising 5% to 20% by Weight of said second slurry, heating said base to dry said second slurry, and subsequently heating said base in vacuum to produce a rough dark heat-radiating surface on said base.

7. A method of providing a heat-radiating surface on a base selected from the group consisting of nickel, iron, ferrous alloys, nickel alloys, and nickel-coated alloys, which comprises coating said base with a rst slurry comprising an organic binder, a solvent for said binder, and a material selected from the group consisting of iron oxide, nickel oxide, and mixtures of iron and nickel oxides, said slurry containing about by weight of said oxide, heating said base to dry said first slurry, reducing said oxide to the metal, sintering said metal to said base, applying to said sintered metal a second slurry comprising an organic binder, a solvent for said binder, and aluminum powder, said comminuted aluminum comprising about 10% by weight of said second slurry, heating said base to dry said second slurry, and subsequently heating said base in vacuum to produce a rough dark heat-radiating surface on said base.

8. A method of providing a heat-radiating surface on a base selected from the group consisting of nickel, iron, ferrous alloys, nickel alloys, and nickel-coated alloys, which comprises coating said base with a lrst slurry comprising an organic binder, a solvent for said binder, and a material selected from the group consisting of iron oxide, nickel oxide, and mixtures of iron and nickel oxides, heating said base at a temperature of 100 to 200 C. to dry said first slurry, reducing said oxide to the metal, sintering said metal to said base, applying to said sintered metal a second slurry comprising an organic binder, a solvent for said binder, and comminuted aluminum, heating said base at a temperature of 100 to 200 C. to dry said second slurry, and subsequently heating said base in vacuum to produce an alitized aluminum-coated surface on said base.

9. A method of providing a heat-radiating surface on steel strip, which comprises coating said strip with a rst slurry comprising iron oxide, heating said strip at about 160 C. to dry said rst slurry, reducing said oxide to iron, sintering said iron to said strip, applying to said sintered iron a second slurry comprising comminuted aluminum, heating said strip to about 160 C. `to dry said second slurry, and subsequently heating said strip in vacuum to produce an alitized aluminum-coated surface on said strip.

10. A method of providing a heat-radiating surface on steel strip, which comprises coating said strip with a rst slurry comprising iron oxide, heating said strip at about 160 C. to dry said first slurry, reducing said oxide to iron, sintering said iron to said strip, applying to said sintered iron a second slurry comprising comminuted aluminum, heating said strip to about 160 C. to dry said second slurry, and subsequently heating said strip in vacuum at a temperature about 725-1000 C. to produce an alitized aluminum-coated surface on said strip.

11. A method of providing a heat-radiating surface on steel strip, which comprises coating said strip with a iirst slurry comprising iron oxide, heating said strip at about 160 C. to dry said first slurry, heating said base in a reducing atmosphere at a temperature of G-1000 C. so as to reduce said oxide to iron, sintering said iron to said strip, applying to said sintered iron a second slurry comprising comminuted aluminum, heating said strip to about 160 C. to dry said second slurry, and subsequently heating said strip in vacuum at a temperature of about 725-1000" C. to produce an alitized aluminum-coated surface on said strip.

l2. A method of providing a heat-radiating surface on steel strip, which comprises coating said strip With a first slurry comprising iron oxide, heating said strip at about 160 C. to dry said rst slurry, heating said base in a hydrogen atmosphere at 975 C. so as to reduce said oxide to iron, sintering said iron to said strip, applying to said sintered iron a second slurry comprising comminuted aluminum, heating said strip to about 160 C. to dry said second slurry, and subsequently heating said strip in vacuum at a temperature of about 725-1000 C. to produce an alitized aluminum-coated surface on said strip.

13. A method of providing a heat-radiating surface on a base selected from the group consisting of nickel, iron, ferrous alloys, nickel alloys, and nickel-coated alloys, which comprises coating said base with a iirst slurry comprising an organic binder, a solvent for said binder, and a material selected from the group consisting of iron oxide, nickel oxide, and mixtures of iron and nickel oxides, said slurry containing 10% to 80% by weight of said oxide, heating said base at a temperature of -200 C. to dry said lirst slurry, reducing said oxide to the metal, sintering said metal to said base, applying to said sintered metal a second slurry comprising an organic binder, a solvent for said binder, and comminuted aluminum, said aluminum comprising 5% to 20% by weight of said slurry, heating said base at a temperature of 100-200 C. to dry said second slurry, and subsequently heating said base in vacuum to produce an alitized aluminum-coated surface on said base.

14. A method of providing a heat-radiating surface on a base selected from the group consisting of nickel, iron, ferrous alloys, nickel alloys, and nickel-coated alloys, which comprises coating said base with a lrst slurry comprising an organic binder, a solvent for said binder, and a material selected from the group consisting of iron oxide, nickel oxide, and mixtures of iron and nickel oxides, said slurry containing 10% to 80% by weight of said oxide, heating said base at a temperature d100-200 C. to dry said first slurry, heating said base in a reducing atmosphere at a temperature of 8001000 C. so as to reduce said oxide to the metal, sintering said metal to said base, applying to said sintered metal a second slurry comprising an organic binder, a solvent for said binder, and comminuted aluminum, said aluminum comprising 5% to 20% by weight of said second slurry, heating said base in air at a temperature of 100-200 C. to dry said second slurry, firing said base in hydrogen to reduce said organic binder, and subsequently heating said base in vacuum to produce an alitized aluminum-coated surface on said base.

References Cited in the le of this patent UNITED STATES PATENTS 2,197,622 Sendzimir Apr. 16, 1940 2,300,400 Axline Nov. 3, 1942 2,328,788 Deputy Sept. 7, 1943 2,539,246 Hensel Jan. 23, 1951 2,541,813 Frisch et al Feb. 13, 1951 2,744,838 Newman May 8, 1956 2,840,493 Okress June 24, 1958 

1. A METHOD OF PROVIDING A HEAT-RADIATING SURFACE ON A BASE SELECTED FROM THE GROUP CONSISTING OF NICKEL, IRON, FERROUS ALLOYS, NICKEL ALLOYS, AND NICKEL-COATED ALLOYS, WHICH COMPRISES COATING SAID BASE WITH A FIRST SLURRY COMPRISING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF IRON OXDE, NICKEL OXIDE, AND MIXTURES OF IRON AND NICKEL OXIDES, DRYING SAID FIRST SLURRY, REDUCING SAID OXIDE TO THE METAL, SINTERING SAID METAL TO SAID BASE, APPLYING TO SAID SINTERED METAL A SECOND SLURRY COMPRISING COMMINUTED ALUMINUM, DRYING SAID SECOND SLURRY, AND SUBSEQUENTLY HEATING IN VACUUM TO PRODUCE A HEAT-RADIATING SURFACE. 